Wireless data communication systems that utilize radio frequency (RF) signals to transmit and receive data are well known. Generally, wireless data communication technology has been applied to high performance long-distance communication systems such as satellite communications or microwave tower telecommunications, or to short-distance local area network (LAN) communication systems, such as a wireless LAN within a home or office environment. In the case of long-distance communication systems, a point-to-point antenna system is required and there must be a line-of-sight transmission path between the transmitter and the receiver. In the case of short-distance wireless LAN communication, an omni-directional antenna system can be utilized and a line-of-sight transmission path is not required because the distances are generally less than a mile. The reason for this difference is due to the fact that RF signals lose power rapidly over longer distances or when transmitting through obstacles such as buildings or walls.
A metropolitan area network (MAN) is a network that can communicate over medium-range distances of between about 1 to 40 miles as would be typically found in providing coverage over an entire metropolitan area. Digital subscriber loops (DSL) services are a good example of a wire-based MAN system that utilizes telephone wires as the communication medium. Cable modem systems are another example of a wire-based MAN system that utilizes coaxial cable as the communication medium. One of the primary advantages of a MAN system is that it allows for higher speed data communications as compared to conventional telephone modem speeds. The primary problem with such wire-based MAN systems is the cost of installing and maintaining the high-quality telephone or coaxial cable communication medium. A fixed wireless MAN system has the obvious advantage of eliminating the costs associated with installing and maintaining a wire based communication medium.
Another advantage of a fixed wireless MAN system is that the wireless communication medium can be designed to provide for higher data communications speeds than conventional wire-based MAN systems. This advantage has caused the fixed wireless MAN systems that have been deployed to date to be designed for ultra high performance and relatively expensive dedicated networks. The market for these fixed wireless MAN systems has been a small number of customers who have high-speed data communication needs that can justify the expense and complicated installation of such systems on an individual basis. As a result of the limited customer base and the need for ultra high performance, the designs of existing fixed wireless MAN systems have developed more along the lines of high performance long distance wireless communication systems.
While there are many factors to consider when designing RF communication systems, some of the more important factors to be considered in designing a fixed wireless MAN system are the assigned frequency, signal modulation and carrier access modulation. Assigned frequency refers to the range of frequencies or oscillations of the radio signal that are available to be used by the system. An example is the assigned band for AM radio signals which operate between 500 KHz and 1600 KHz. Signal modulation refers to the way in which information or data is encoded in the RF signal. An example is the difference between amplitude modulation (AM) radio signals and frequency modulation (FM) radio signals. Carrier access modulation refers to the way in which the assigned carrier frequencies are used to carry the RF signal. An example is the difference between using a single wide channel or multiple narrow channels over the same assigned frequency bandwidth.
For purposes of this invention, the design of a fixed wireless MAN system is focused on frequency ranges less than 10 GHz. Other medium-distance wireless communication systems have been developed, such as the local multipoint distribution system (LMDS) that operate at much higher frequency ranges, such as 28 GHz to 31 GHz. These higher frequencies are subject to different technical concerns and require larger external antenna systems that provide line-of-sight transmission paths from the top of one building to another.
Because of the desire for higher data speeds, all of the existing fixed wireless MAN systems have utilized more complicated schemes for signal modulation. To support faster speed downstream transmissions, these systems typically use a 16-bit quadrature amplitude modulation (QAM) or 64-bit QAM to transmit downstream from the base station to the CPE at a data rate of at least 10 Mbps.
Unlike the many fixed wireless LAN systems that have been developed for short-distance communications and use a spread spectrum form of carrier access modulation that spreads one signal across the assigned frequency bandwidth, the relatively few fixed wireless MAN systems that have been developed to date have utilized multi-carrier modulation as their carrier access modulation. In multi-carrier modulation, the signal is divided into several parallel data streams and these parallel data streams are simultaneously sent along different slower speed channels and then reassembled at the receiver to produce a higher effective transmission rate. The multi-carrier modulation scheme that has been designated by the IEEE standards committee to be used as the extension to the 802.11 wireless LAN standard for high-speed wireless data communications is known as orthogonal frequency division multiplexing (OFDM). The OFDM modulation scheme makes for a more efficient use of the assigned bandwidth and improves the ability to receive higher speed transmissions.
All of these more complicated modulation schemes for the existing fixed wireless MAN systems generally require more expensive equipment and more transmission power at each base station. To capitalize on the increased investment associated with each base station, existing fixed wireless MAN systems have been designed to minimize the number of base stations required to provide coverage for a given area. The radius of a typical coverage area for existing wireless man systems ranges between 10 to 30 miles.
Larger coverage areas are also used to minimize the need to reuse the same frequency channels in adjacent coverage areas. Because higher transmission powers are used to transmit at the higher data rates in all of the existing fixed wireless MAN systems, the higher power signals prevent the reuse of the same frequency channels in adjacent coverage areas and can even preclude the reuse of the same frequency channels at distances up to three to five times the radius of the coverage area. Consequently, larger coverage areas reduce the impact of problems caused by the inability to reuse frequencies in adjacent coverage areas.
The most significant disadvantage of larger sizes for the coverage area for each base station is the greater potential for signal loss or attenuation between the base station and the CPE. To counteract this potential signal loss over the larger distances and to improve reception at the higher power, higher transmission speeds, all of the existing fixed wireless MAN systems utilize a point-to-point antenna system that requires a line-of-sight transmission path between the base station and an externally accessible antenna that is connected to the CPE. For example, see the prior art fixed wireless MAN system configuration of FIG. 1 wherein the CPE within a single-user environment, e.g., a home, is connected to an antenna that is to the exterior of the single-user environment and where within a multi-user environment, e.g., a small office, each CPE is connected to its own antenna that is located exterior to the multi-user environment.
Given the relatively limited customer base and the need for all ultra high-performance that has dictated the development of existing fixed wireless MAN systems, the use of externally accessible antenna that provide a line-of-sight transmission path is both necessary and understandable. It will be desirable, however, to provide for a fixed wireless MAN system that does not require the use of an externally accessible antenna and could be more broadly deployed to provide higher data speeds more effectively to a larger number of consumers.